Heat-shrinkable plastic films are used for labelling, protection, parceling, and wrapping of glass, metal, ceramic and plastic bottles, boxes, cans, pipes, and wood. For example, heat-shrinkable films are widely used as tamper evident labels on food and pharmaceutical products and as primary labels on plastic containers such as soft drink bottles. Shrink films are classified into two categories: (1) biaxially oriented film for wrapping wherein the film shrinks in both the x- and y-axis directions, and (2) uniaxially oriented film wherein the film primarily shrinks in the stretched or oriented direction and has a little shrinkage in the unstretched or nonoriented direction.
The most widely used shrink film is polyvinylchloride (PVC), however, shrink films have been prepared from polystyrene (OPS), oriented polyethylene, oriented polypropylene, and polyesters. While PVC has good heat-shrinkable properties, PVC is not suitable for low temperature shrink packaging. In addition, the incineration of PVC generates hydrogen chloride which causes corrosion of the incinerator and environmental pollution.
Example of a heat-shrinkable polyester film are disclosed in U.S. Pat. Nos. 4,996,291 and 4,963,418. U.S. Pat. No. 4,996,291 discloses a heat-shrinkable polyester film prepared from polyethylene terephthalate. While a list of additives is included, plasticizers are not listed. U.S. Pat. No. 4,963,418 discloses a heat-shrinkable polyester film prepared from a polyester, a polyester copolymer, or a mixture of a polyester and a polyester copolymer. No suggestion is made of forming a blend with a plasticizer or any other additive.
Previously disclosed polyester films have at least two major disadvantages, a higher on-set shrink temperature than 50.degree.-60.degree. C. and a higher shrink rate than 4% per degree celsius. A high on-set shrink temperature means that the film or sheet requires a higher temperature at which the film starts to contract in a heated shrink tunnel or oven. High on-set temperature can cause container or content damage. The other disadvantage is a higher shrink rate which refers to the percent shrinkage per degree of temperature increase. A high shrink rate causes wrinkle or bubble formation in shrink film or sheet as it travels through the shrink tunnel.
The present inventors have unexpectedly determined that the on-set shrink temperature and the shrink rate of a heat-shrinkable film or sheet prepared from a polyester can be decreased by melt blending a critical amount of a plasticizer selected from a C.sub.4 to C.sub.20 alkyl ester of an epoxidized fatty acid having 12 to 20 carbon atoms provided the plasticizer has sufficient stability to permit its incorporation into the polyester at a temperature of 200.degree. C. to 300.degree. C. Moreover, the polyester/plasticizer blends of the present invention produce clear film without the environmental disadvantages associated with other films.